In order to transfer experience from one waterwork to another, it is helpful to know (i) the spontaneous ozone requirement, (ii) the lifetime of the ozone, and (iii) the oxidation efficiency of the secondary oxidants derived from decomposed ozone (OH* radicals) in the different waters. The spontaneous ozone requirement of surface and ground waters (0.1 to 1 mg/l) and the lifetime of the ozone (30 to 2,000 sec at pH 8) can easily be measured. The effect of the secondary oxidants (OH* radicals) can best be characterized by following the elimination rate of the individual suitable referenced micropollutant as a function of the amount of ozone decomposed. This way the “Oxidation-Competition Value”, Ω, of the water can be determined. This Ω is a linear sum of the concentrations of all impurities which consume OH* radicals, multiplied by the individual “Oxidation-Competition Coefficient.” The Ω values have been determined for 40 different types of waters. For the elimination of benzene (reference solute) to its 37% value, the required amount of ozone decomposition (e.i. Ω {benzene}) varied in lakewater from 0.5 mg/l (mesotrophic lake) to 2.2 mg/l (highly eutrophic lake) and in groundwater from 0.3 mg/l (soft mountain groundwater) to 2 mg/l (medium loaded groundwater).